I compiled a list of skill affixes for better readability. All of them are datamined, so they may be inaccurate. I only included the highest level affixes on the list.
All of the skill bonuses are suffixes, so only one can spawn on a Magic item. Rare items are a question mark in this regard, because we don't know enough about the item generator. It's possible, that the number of skill affixes is limited to one even on Rare items.
These affixes start appearing on class-specific items in Nightmare and on generic items in Hell. What's interesting is that some of the highest level affixes can't spawn on the most powerful class-specific items, because their item level isn't high enough, but they can spawn on the generic items with the highest level. I hope this will change.
Code:Item Group 1: Voodoo Masks, Helms Item Group 2: Shields, Mojos, Bows, Crossbows Item Group 3: Daggers, Spears, Bows, Crossbows, Ceremonial Knives, 2H Axes, 2H Swords, 2H Maces, Polearms Skill Bonus May Spawn on Item Group Acid Cloud +2-8% Critical Chance 1 Corpse Spiders +5-14% Inreased Damage 2 Firebomb Mana Cost Reduced by 1-9 3 Firebats Mana Cost Reduced by 4-12 1 Haunt +5-14% Inreased Damage 3 Locust Swarm +5-14% Inreased Damage 1 Plague of Toads +5-14% Inreased Damage 3 Poison Dart +5-14% Inreased Damage 2 Spirit Barrage +5-14% Inreased Damage 2 Wall of Zombies Cooldown reduced by 1-3 seconds 2 Zombie Charger Mana Cost Reduced by 4-12 2 Summon Zombie Dogs Cooldown reduced by 1-9 seconds 1